Our objective is to describe the influence of maternal and fetal nutritional and endocrine status on the placentaltransport and fetal metabolism of calcium and other substances. Four types of studies are underway. All use vascular catheters in the fetus. (1) Maternal and fetal plasm concentrations of Ca ions, total Ca, Mg, Na, K, Cl and Pi together with pH and pCo2 and sometimes parathyroid hormone have been measured in cattle and sheep. (2) Double tracer experiments using Ca-45 and Ca-47 have been conducted on cattle and sheep. The data are being used to build a model of the maternal-placental- fetal calcium system. (3) Attempts have been made to measure the transplacental potential difference with specially constructed "salt bridges" in place for up to 10 days. (4) Studies have been made of fetal metabolism of 1-131 labeled parathyroil hormone and of fetal plasma parathyroid hormone responses to EDTA infusion. These studies are beginning to suggest that the rate of placental supply of calcium to the fetus may influence the ability of the neonatal animal to adapt to the sudden cessation of this supply.